Thick positive cathodes are good for creating energy-rich thin-film batteries. A thick positive cathode substantially increases the active cathode mass per unit area. Unfortunately, producing such cathodes with typical vacuum vapor phase processes has been problematic.
Cathodes made with a typical vacuum vapor phase method have a number of limitations. For instance, vacuum vapor phase deposited materials typically grow in columns as schematically shown in FIG. 1. This figure depicts schematically and in cross-sectional view three microscopic columns, grown by a vacuum vapor phase deposition method, of the positive cathode layer of an electrochemical device. As the columns grow through the process, the bases of these columns remain anchored to the substrate surface and the cross sectional area of these bases remains virtually fixed as the height of the columns grows. As the height of the columns increases, the aspect ratio (height of column/width of column) increases and the cathode film consisting of these columns and thus the entire device becomes mechanically unstable, typically around an aspect ratio of 15. Thus, there are limitations to the height, and therefore the thickness, of columns grown with a vacuum deposition processes. Limitations on the height directly correspond to the thickness of the cathode and the energy of an electrochemical device per unit area that can be produced using a vacuum vapor phase deposition method. Furthermore, thick cathodes take a relatively long time to grow using a vacuum vapor phase process and are, therefore, quite expensive. For instance, LiCoO2 positive cathodes grown in a vacuum vapor phase deposition method above about 3 μm become overly expensive because of their long deposition time.
Thus, there is demand for electrochemical devices whose cathodes can be produced thick and reliably while being fabricated quickly and inexpensively. Further, it would be desirable to accomplish these demands using any of the many well-known non-vapor phase deposition techniques and processes, such as slurry coating, Meyer rod coating, direct and reverse roll coating, doctor blade coating, spin coating, electrophoretic deposition, sol-gel deposition, spray coating, dip coating, and ink-jetting, to name a few.
Depositing a thicker cathode in order to increase the energy of an electrochemical device per unit area results in an increased, overall thickness of the device. Because an overall thickness increase of a milli, micro, or nano device is typically undesirable, the device manufacturer has to explore options of how to compensate for or offset such a thickness increase. A generally valid and desirable approach is to minimize the thickness and volume of all of the non-energy providing components inside an electrochemical device.
One of the options is to reduce the non-energy providing packaging of an electrochemical device. Both the encapsulation and the substrate are inherent and usually large, fractional parts of the packaging.
For instance, the reduction of an encapsulation thickness from 100 micrometers, which is a typical thickness for a laminate encapsulation, to a true thin-film encapsulation in the range of 1-10 micrometers would allow the electrochemical device manufacturer, for example, to increase the thickness of the energy bearing cathode by almost 100 micrometers without any discernible overall thickness change of the device. This design approach substantially improves the volumetric quantities of energy, capacity, and power of the electrochemical device. Because these physical performance quantities are required to be delivered in the smallest volume possible for most any milli, micro, or nano electrochemical device, the reduction of the non-energy providing components inside an electrochemical device is critically important for its acceptance in the marketplace.
The other option is to fabricate an electrochemical device onto the thinnest possible substrate, if used, traded or sold as a standalone device. This is different from the non-standalone case wherein the device manufacturer may exploit an existing, free surface in an electronic device (chip surface, printed circuit board surface, etc.) and then directly integrate, fabricate or deposit the electrochemical device onto that free surface. This surface then serves as the electrochemical device's substrate as well. One may consider such an electrochemical device being configured with a zero-thickness substrate because no further substrate thickness was introduced by the electrochemical device into the final electronic device. In the more common, standalone case, however, the limits of substrate thinness are reached when it does not provide adequate chemical and physical, mainly mechanical, protection or functionality anymore to support the electrochemical device. Because most vacuum deposited cathode materials require high-temperature processing to fully develop all of their physical properties, which in turn creates film stresses that are translated into the substrate, the mechanical properties of these vacuum vapor deposited cathode materials may challenge any substrate in terms of mechanical deformation.
The typical result of vacuum vapor phase deposited films in conjunction with high-temperature processing is a bending, warping, or general deformation of the substrate and thus the entire electrochemical device. If this situation occurs, then completing the fabrication of the electrochemical device becomes difficult, in addition to the mere fact that a deformed electrochemical device is not well suited for device integration. In contrast, non-vapor phase deposited cathode materials may be fabricated with most or even all of their important physical properties already developed at the time of deposition, so that any high-temperature processing becomes redundant. Hence, non-vapor phase deposited cathode materials and other components of an electrochemical device create less stress in the substrate and allow the use of a thinner substrate without the risk of substantially deforming it.
Accordingly, there is also a need for capsulation that exhibits fairly high-temperature characteristics.
Thus, there is demand for an electrochemical device (i) whose cathode can be produced thick and reliably while being fabricated quickly and inexpensively, (ii) whose substrate thickness is as thin as possible while not being deformed by the component layers of the electrochemical device, (iii) whose encapsulation is fabricated as thin as possible while still providing adequate protection against the ambient in which these devices are operated, and/or (iv) whose encapsulation is composed of high-temperature materials that provide the entire electrochemical device with increased thermal resilience.